The invention relates to a process and a device for melting glass starting with vitrifiable materials, a device more commonly called a melting furnace, continuously to feed melted glass to forming equipment for flat glass such as rolling or float equipment, or for hollow or cut glass like numerous forming machines, or for insulation glass (glass wool, rock wool), or for glass for reinforcement fibers or even special glass for television screens or others.
The invention can be applied to all types of melting furnaces having production capacities for melted glass that can range, for example, from batches of 10 tons/day up to 1,000 tons/day and more.
This type of furnace usually consists, as is known, of a series of compartments emptying into one another and each having specific functions and dimensions. The furnace must indeed be able to melt vitrifiable materials and guarantee the chemical and thermal homogeneity of the glass once it is melted.
Furnaces can be placed in two broad categories according to the heating method used to melt the vitrifiable materials in the melting compartment:
on the one hand, there are electric melting furnaces, called "cold vault," where melting is performed by electrodes that are immersed in the depth of the melted glass, which is known, for example, from patent EP-B-0-304 371.
On the other hand, there are draft furnaces, also called regenerative furnaces, known notably from U.S. Pat. No. 4,599,100. In this case, the heating power is provided by two rows of burners generally operating with a fuel/air mixture, and in alternation; the combustion gases then alternately reheat one or the other of the two regenerators placed opposite one another on both sides of the melting compartment and connected to the latter. The combustion gases are thermally exhausted by stacks of refractories that comprise these regenerators, refractories that then return the heat thus stored to the melting compartment. This method of heating is efficient and widely used, although it is not without certain inherent drawbacks. Thus the energy cost of fuel/air burners is relatively high. Besides, the operating system of burners that are "activated" alternately with cycles on the order of 15 to 60 minutes is not the most simple to control rigorously and can cause a discontinuity in continuous manufacturing and the equilibrium of the temperatures is affected. Their use further leads to introducing, into the melting compartment, a significant amount of air, thus of nitrogen, from which there is an increased risk of seeing form a certain amount of polluting gases of the NO.sub.x type that must then be treated.
Further, the large amount of special and costly refractories necessary for manufacturing regenerators significant raises the price of construction of the furnace.
The object of the invention is thus to mitigate the drawbacks connected with the use of draft furnaces by proposing a new type of draft heating that greatly reduces the energy cost and the material cost for construction of the furnace, that simplifies its operating method, while guaranteeing that melted glass of at least as high a quality is obtained.
The invention also has the object of reducing the wear of the refractories comprising the walls of the furnace and thus of increasing the lifetime of the furnace.
The object of the invention is a furnace for melting vitrifiable materials, a furnace comprising a melting (or melting/refining) compartment for glass equipped, in the upstream part, with at least one opening intended to be fed with vitrifiable materials with the help of charging devices placed opposite said opening. In the downstream part, said melting compartment comprises at least one exit opening for melted glass emptying into one or more successive downstream compartments intended to convey melted glass toward the forming zone. According to the invention, the melting of vitrifiable materials is performed, in the melting compartment, essentially by at least one fuel oil and/or gas burner, the combustive material being formed mainly of essentially pure oxygen, at least 50% of the combustive material necessary for the desired combustion being supplied separately by at least one oxygen lance. Essentially pure means, according to the invention, at least 80% and preferably at least 90% oxygen. Separately means separate arrival points for the fuel oil and/or gas on the one hand and for the oxygen on the other hand. Fueled intended for use with the operation of the burners are those typically used for glass melting furnaces including fuel oil and gas.
In the framework of the invention, the terms "upstream" and "downstream" refer to the overall direction of flow of the melted glass through the furnace. Melting compartment means the melting compartment and the melting/refining compartment.
To choose, according to the invention, a method of heating combining burners and oxygen lances supplying at least 50% of the oxygen separately indeed offers a whole series of advantages compared to more conventional burners operating notably with an air-type combustive material, or even compared to oxygen burners supplied with a stoichiometric amount of oxygen.
This method of heating with oxygen makes it possible to eliminate, first of all, the traditional operation by "inversion" for draft furnaces: the oxygen burners can maintain a constant operating regime over time, which makes the use of the furnace simpler, this continuous operation is more regular and makes it possible to have much more fine adjustments than with operation by inversion. Especially the presence of regenerators made of stacks of costly refractories, susceptible to wear, can be completely eliminated. The oxygen burners thus are able to heat the vault of the melting compartment and the so-called "heating chamber" volume between said vault and the plane of melted glass, continuously and without having to use regenerators.
The atmosphere that prevails above the plane of the glass in the melting compartment is much more stable and controlled, which can prove to be significant for the production of so-called special glasses.
Further, the thermal efficiency of this type of burner is notably higher than that of conventional burners operating with an air-type combustive material, because of the absence of nitrogen, which considerably decreases the volume of fumes generated. Thus considerable reductions in energy cost are obtained overall, and this type of burner makes it possible also to envision considerable increases in the specific draw of the furnace.
The fact that the burners chosen according to the invention introduce a very small amount, almost none, of air into the melting/refining compartment very greatly reduces the possibilities for forming NO.sub.K type polluting gases, making it all the more less costly to treat the combustion gases exhausted from the compartment.
Further, always with respect to conventional burners, the oxygen burners make it possible to introduce, into the melting compartment, a much smaller volume of gas and, similarly, the volume of gas resulting from the combustion is likewise greatly reduced as already mentioned. This means that it can be envisioned to reduce the volume of the so-called "heating chamber" mentioned above, notably, for example, by slightly lowering the vault of the melting compartment which, here again, tends to reduce simultaneously the energy cost and the construction cost of the furnace itself.
Everything, thus, in using oxygen burners operating without inversion contributes to achieving a more reliable furnace, less costly in its design and one that makes it possible to have energy savings that can go up to well beyond 15% with respect to a conventional draft furnace of similar dimensions.
However, when essentially pure oxygen is used as a combustive material in fuel oil and/or gas burners with so-called stoichiometric amounts, the flame temperature obtained at the plane of the exit of the burners is higher than the flame temperature using air as the combustive material. A rapid deterioration of the refractories comprising the walls of the furnace, notably around the tip of the burners, can thus result.
An object of the invention is thus a process for melting vitrifiable materials in a melting compartment, continuously to feed melted glass to glass forming equipment in which the melting of vitrifiable materials is performed essentially by the combustion of a fuel oil and/or gas mixture with essentially pure oxygen, the supply of fuel oil and/or gas to the melting compartment being performed at least at one point that has a deficiency of oxygen with respect to stoichiometric amounts, at least 50% of the oxygen corresponding to the total combustion being supplied separately at least at one different arrival point.
This supply of at least 50% of the oxygen necessary for total combustion by at least one different arrival point and preferably by several different arrival points further yields a greater flexibility for regulating the temperatures in the melting compartment, the zones(s) of the compartment where the oxygen is supplied generally corresponding to the point at which it is desired to control the temperature level.
Further, the feed of the burners with a deficiency of oxygen, according to the invention, and the supply of oxygen, in combination, by wisely distributed oxygen lances, notably makes it possible to have a controlled flame temperature called "low temperature, low NO.sub.x flame." Further, the fact of inserting oxygen between the flame and the walls of the furnace assures an oxidizing atmosphere at the walls. The flame is also at a distance from the walls and thus the phenomenon of exudation and attack on the refractories in their vitreous phase is decreased.
The combination of oxygen burners fed with a deficiency of oxygen and oxygen lances results in a better distribution of the flame over the mixture and the glass bath. This better distribution of the flame makes it possible to lower the vault temperatures and to increase the temperatures of the bottom. Finally, this makes it possible likewise to envision increases in the specific draw of furnaces of 10 to 30%.
As indicated above, the supply of oxygen by burners on the one hand and by oxygen lances on the other hand, is made thus with 0 to 50% only by burners and the rest, to 100% of the oxygen necessary for combustion, by oxygen lances.
Preferably, the oxygen supply is made with at least 80% separately by the oxygen lances.
Finally, all or essentially all the oxygen can be advantageously supplied separately. Thus any incident at the tip of the injector (burner) in the case of fouling or run-out, in particular in the case of special glasses, is avoided. All or essentially all the oxygen is meant to mean 100 to about 95% of the oxygen.
According to one of the characteristics of the invention, the flame of a burner is fed by successive feeds of oxygen to the flame and at certain locations in the furnace to have the desired flame temperature.
However, the very favorable assessment described above would be compromised if, contrary to the invention, the entry of air into the melting compartment, coming from downstream compartments, were not avoided. In the opposite case, indeed, the risk is run of recreating a certain amount of NO.sub.x type polluting gases in the melting compartment and the saving realized in terms of energy in this compartment is decreased considerably. The arrival of air can be prevented with the help of sealing means for the gases of the melting/refining compartment with respect to the rest of the furnace. The or these means of sealing thus "insulate" the atmosphere prevailing above the glass melted in the melting compartment from the atmosphere of the successive downstream compartment(s) that are adjacent to it. These "downstream" compartments are intended to condition the glass, i.e., are essentially intended to progressively cool it until it reaches its forming temperature, to perfect its chemical and thermal homogeneity and to eliminate foreign bodies of the unmelted type or particles of refractory material. Now this thermal conditioning can be performed in one or the other of these so-called downstream compartments by using, as is known, alternatively or in combination, reheating means, for example conventional fuel-air burners, and cooling means that introduce the air in a large amount at ambient temperature into these compartments. Thus it is necessary to prevent these types of gases from "returning" toward the melting/refining compartment so they do not disturb its very controlled atmosphere.
Of course if the downstream compartment(s) are designed so that, for example, they use cooling means without the introduction of air, and have an atmosphere not composed of air, these sealing means are no longer indispensable.
According to an embodiment of the device according to the invention, the burners or injectors are distributed in rows by being alternated with oxygen lances in the upstream and/or downstream wall, and or the lateral walls parallel to the glass bath, with a number and an inclination of +5 to -15 degrees, depending on the structure and dimensions of the furnace, and on the position with respect to the glass bath. The burners and oxygen lances empty into the melting/refining compartment through superstructure walls whose sections can be very reduced and do not alter the thermal insulation of the unit. The burners can be independent or gathered in groups of burners whose heating power is regulated independently from one group to the next. The groups are placed essentially perpendicular to the axis of the furnace on the lateral walls and essentially parallel to the furnace on the upstream and downstream front walls, with the possibility each time of being oriented from 0 to 20 degrees with respect to these axes and to the horizontal plane. The heating can be modulated and regulated in an optimal way in the entire melting/refining compartment and can achieve all the desired temperature profiles, according to the types of melted glass and fabrication. Other arrangements of burners and oxygen lances are of course possible.
There can be provided, in the melting/refining compartment, mechanical means of the babbling type to accelerate the convection or auxiliary heating means of the type where electrodes are- immersed in the glass to adjust or correct the temperature profile.
The fact of having eliminated the recuperator or the regenerators reduces the number of openings, frees the entrance and the space around the furnace, which makes it possible better to insulate it and facilitates its maintenance.
To recover, to the maximum, the heat energy from the fumes coming from the combustion of the burners in the melting compartment, preferably the exhaust openings are placed behind the openings for feeding the vitrifiable materials, the fumes following a path from the center of the furnace toward the walls (making a screen between the flame and the walls) to return from downstream toward the upstream to enter above the charging zone where the vitrifiable materials are floating on top, which can thus advantageously be preheated.
Several positions can be adopted for the feed opening(s) for vitrifiable materials. The latter can be made, on the one hand, in the one or two lateral walls, on the other hand, in the front wall of the melting compartment.
An advantageous solution can be two symmetrical openings opposite one another in the lateral walls.
It is possible to provide the auxiliary exhaust openings for the fumes either in the upstream front wall or in the lateral walls of the melting/refining heating chamber.
These fumes, once they exit the heating chamber of the furnace, are still relatively hot, it can be provided to convey them into heat recovery devices or devices for preheating vitrifiable materials before they are charged. They can still be used to preheat oxygen.
Another advantage of the invention using the combination of gas and/or fuel oil burners (or injectors), fed with a deficiency of oxygen, this deficiency being able to be total, and oxygen lances, is a more flexible regulation of the flame and notably a regulation of its position with respect to the glass mixture, the flame being drawn toward the location where the oxygen is supplied.
This advantage, notably, makes it possible to design variants of the device according to the invention in the arrangement of burners and oxygen lances.
Separate supply of oxygen generally means, according to the invention, a supply of oxygen at one or more points that are at least 5 cm and preferably at least 10 cm away from the point at which the fuel oil and/or gas is supplied, i.e., from the tip of the burner. The oxygen can advantageously be supplied at two points around each burner, and preferably at a level lower than the level at which the fuel oil and/or gas is supplied.
Thus the invention envisions likewise a unit that can be used in the heating system combining burner and oxygen lances according to the invention. This unit comprises a refractory block equipped with a passage for emplacing a burner, this passage being flared notably in the form of a truncated cone toward the face intended to be oriented toward the interior of the furnace, and two passages placed at equal distances and at more than 5 cm and preferably at more than 10 cm from the axis of the burner passage, these two passages intended for the emplacement of two oxygen lances being further located at a level lower than the level of the burner. The unit can be equipped with a burner and with two oxygen lances.
This arrangement of oxygen lances with respect to the burner makes it possible regulate the flame very well while notably avoiding excessive heating of the refractory where the burner tip is located.
Other characteristics and advantages of the invention come out of the following description with the help of the figures that represent: